VARIETIES 01 OBSIDIAN*. 
101 
thrown cut in a softened state, and had afterwards been sub- 
ject to a rotary motion. They contain a quantity of vitreous 
feldspar, of a snow-white colour, and the most brilliant 
pearly lustre. These obsidians are, nevertheless, but little 
transparent ou the edges; they are almost opaque, of a 
brownish black, and of an imperfect conchoids! fracture. 
J hey pass into pitch-stone ; and we may consider them as 
porphyries with a basis of obsidian. The second variety is 
found in fragments much less considerable. It is in general 
°t a greenish black, sometimes of murky gray, very seldom of 
a perfect black, like the obsidian of Ilecla and Mexico. Its 
fracture is perfectly conchoidal, and it is extremely transpa- 
rent on the edges. ’ 1 have found in it neither amphibole nor 
pyroxene, but some small white points, which seem to be 
feldspar. None of the obsidians of the Peak appear in those 
gray masses of pearl or lavender-blue, striped, and in sepa- 
rate wedge-formed pieces, like the obsidian of Quito, Mexico, 
and Lipari, and which resemble the fibrous plates of the 
rrystalites of our glass-houses, on which Sir James Hall, 
Hr. Thompson, and M. de Bellevue, have published some 
curious observations.* 
l'he third variety of obsidian of the Peak is the most 
remarkable of the whole, from its connexion with pumice- 
stone. It is, like that above described, of a greenish black, 
sometimes of a murky gray, but its very thin plates alternate 
with layers of pumice-stone. Dr. Thomson’s fine collection 
a t Naples contained similar examples of lithoid lava of Ye- 
?p} vius , divided into very distinct plates, only a line thick. 
I lie fibres of the pumice-stone of the Peak are very seldom 
parallel to each other. and perpendicular to the strata of 
obsidian; they are most commonly irregular, asbestoidal, like 
ibrous glass-gall ; and instead of being disseminated in the 
obsidian, like crystalites, they are found simply adhering to 
one ot the external surfaces of this substance. During my 
I s ay at Madrid, M. llcrgen showed me several specimens 
ln t ‘ le miueralogical collection of Don Jose Clavijo ; and for 
, ''j name crystalites has been given to the iiyslalized thin plates 
b S r)T' iU ®* ass c0 °bng slowly. The term glastenized glass is employed 
wl l] 1 *' Thom l ,so " ant ^ others to indicate glass which by slow' cooling is 
>olly unvitrified, and has assumed the appearance of a fossil sub- 
stance, or real glass-stone. 
